pgsql: Add regression test coverage for contrib/pg_buffercache.

Add regression test coverage for contrib/pg_buffercache.

We can't check the output of this view very closely without
creating portability headaches, but we can make sure that
the number of rows is as-expected.  In any case, this is
sufficient to exercise all the C code within, which is a
lot better than the 0% coverage we had before.

DongWook Lee

Discussion: https://postgr.es/m/CAAcByaLCHGJB7qAENEcx9D09UL=w4ma+yijwF_-1MSqQZ9wK6Q@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/be39d88934331c47c43d8c51500305e928f06240

Modified Files
--------------
contrib/pg_buffercache/.gitignore                  |  4 ++++
contrib/pg_buffercache/Makefile                    |  2 ++
contrib/pg_buffercache/expected/pg_buffercache.out | 10 ++++++++++
contrib/pg_buffercache/sql/pg_buffercache.sql      |  6 ++++++
4 files changed, 22 insertions(+)